Earlier quoted context omitted.
The bigger problem for the studios is that if word gets out that a blockbuster movie fails the expectations of its potential audience. Surely advance screenings and reviews have exactly the same effect in that case.
Word of mouth from your fellow franchise fans might have a bigger influence than Roger Ebert dissing the movie. Also, there's such a thing as a review embargo. It would be interesting to see how review embargos correlate with box office success and after-the-fact ratings.
If the studios think they have a real turkey on their hands (and they'll know this before the critics see it - the focus groups will have told them if they didn't know just by watching themselves) they tend to drop all press screenings.
But you're right that word of mouth will often be more significant than a review. Blockbuster type films tend to be completely critic proof - the reviewers can savage them as much as they like and there's little impact - but if a couple of friend's whose opinions you trust tell you a film sucks there's a good chance you'll steer clear.